1. Field of the Invention
This invention relates to microlithography, and more particularly to alignment of masks/reticles to wafers/substrates and to measurement of overlay.
2. Description of Related Art.
In microlithography, difficulties exist in accurate measurement of overlay (O/L), which comprises the degree of misalignment between successive layers of patterns on a thin film electronic structure and in aligning the masks/reticles used to print such layers to preceding layers. From a slightly different point of view, difficulties exist in accurate estimation of centerlines in the context of optical alignment and overlay (O/L) measurements. Inaccuracies of O/L measurement and alignment are introduced by asymmetrical, nonuniform coating of resist in deposition (spin-on is typical) over O/L and alignment marks/targets. For example, spin-on deposition of resist yields peaks and valleys in the resist which are asymmetric in reference to underlying topography. As a result, images of such marks below are themselves asymmetric, making estimation of centerlines ambiguous and inaccurate.
Nakata, U.S. Pat. No. 4,906,852 uses a variable path length delay to offset the phase of one signal in reference to another. The first is that of a wafer alignment mark and the second is that of a planar area in proximity. Spatial and temporal coherence of light are essential to achieve strong isolation of the reflection off the bottom of photoresist. The assumption that reflection being the strongest one is made and is also crucial for operability. A mechanical motion or the equivalent to vary the length of the delay path of the Mach-Zehnder interferometer is required.
Starikov, IBM Technical Disclosure Bulletin, Vol. 33, No. 5 (Oct. 1990) pp 114-5 "Structures for Test of Asymmetry in Optical Imaging Systems" discusses testing asymmetry of optics for optical lithography, alignment, and measurement of overlay and size.
Studies of sensitivity of various alignment systems to asymmetry in photoresist coverage, such as Chris P. Kirk, "Theoretical Models for the Optical Alignment of Wafer Steppers", SPIE, Vol. 772, Optical/Microlithography VI (1987) pp. 134-141, (referred to hereinafter as Kirk '87) exist suggesting errors up to 1 .mu.m for some bright field monochromatic alignment systems.
For one such system considerable errors of alignment have, in fact, been reported, for example, K. A. Chivers, "A Modified Photoresist Spin Process for a Field-by-Field Alignment System" Kodak Microelectronics Seminar, Proceedings Oct. 29-30, 1984, San Diego, Calif., pp 44-51.
Some makers of alignment systems ("Wanta '87": M. D. Wanta et al, "Characterizing New Darkfield Alignment Target Designs", KTI Microelectronics Seminar, Interface '87, Proceedings Sponsored by KTI Chemicals, Inc. San Diego, Calif., (Nov. 19-20 1987) pp. 169-181) and ("Lambson '91": C. Lambson and A. Awtrey, "Alignment Mark optimization for a Multi-Layer-Metal Process", KTI Microlithography Seminar, Interface '91, Proceedings Sponsored by KTI Chemicals, Inc. San Diego, Calif., (Oct. 14-15 1991) pp. 37-52) have considered the use of signal symmetry as a measure of goodness of alignment targets, when the asymmetry is due to the process of mark formation, such as in metallization levels.
In the case of alignment by commercially available systems there is a difficulty with the wafer alignment marks, whose centerline is estimated in reference to the mask/reticle marks.
FIG. 1 shows a sectional view of a substrate 10 in the form of a wafer with a wafer alignment mark 12 illustrated here in the form of a hollow space (which alternatively can be a rise) on the upper surface 11 of substrate 10, which extends along the x axis of the drawing. The illustrative example of a mark 12 is about 15 .mu.m wide and about 0.37 .mu.m deep and coated with a layer of photoresist 14 about 1 .mu.m thick along the y axis which at the time of spin-on deposition was flowing from right to left, parallel to the x axis, as indicated by arrow 13. Some of the photoresist 14 is deposited within the depression formed by the mark 12, leaving a depression 16 in the upper surface 18 of the photoresist 14. It can be seen that the heights of the photoresist 14 are different, at level 17 at the leading edge 50 and at level 15 (a little higher than level 17) at the trailing edge 52 directly above the leading and trailing edges 19 and 21 of mark 12. The resulting difference is in mark reflectivity over the leading and trailing edges is the source of the image asymmetry in bright field viewing of the mark.
FIG. 2 shows a modification of FIG. 1 with a different mark 112 on substrate 110 only 1 .mu.m wide with the thickness of the resist 14 the same and the depth of the mark 112 the same as mark 12. Note that the depression 116 in the upper surface 118 of the resist 114 over the mark 112 is much smaller than that over mark 12, indicating better planarization. Also, notice the apparent improvement in symmetry of the resist top surface over the mark.